Well, this is also discussed at the linked TMO topic, as well as here at Together.Jolla.
At TMO Bundyo has created RPM package, which ease the install process. I guess other layouts can be packaged easily like that.

However, about Jolla Shop, there is no good news, in an other topic hereStskeeps answered that "To the question: no, not currently: you need to edit layouts.conf as said, and you can only install into /usr/share/app-name/"

Thus, currently, VKB could be packaged and installed with RPM, but cannot be added to Jolla shop due to policy reasons.

I have made emoji keyboard to Jolla with font support too. Unfortunately harbour won't accept this, at the moment and thus this is available from Openrepos repository only. I am offerin the solution as free software for Jolla to include into Sailfish OS as is or modified if they wish.

Note: SailfishOS has a bug in Unicode UTF-8 implementation and some individual OS and Jolla applications and libraries do not transfer and/or encode correctly over two (2) byte wide Unicode UTF-8 encoded characters. SMS sending is one of them.

Comments

Thank you, this is great. But it also highlights one big limitation of the current harbour/development state.
This is something that adds basic functionality to the messaging application, does not need to pollute the launcher with extra applications and cannot be easily distributed. Part of this should probably go in the Mer layer.... We may have better luck with that?

I am fine if someone takes and incorporates this to Mer. The whole keyboard concept should be fixed so that new keyboards can be added without breaking package dependencies. Currently layouts.conf is single flat file and adding new keyboard layout requires changes to this system file.

I found existing method how to configure custom add-on keyboards without need to hassle with layouts.conf. There can be multiple '.conf' files and those are processed in alphabetical order of filename.